1. Field of the Invention
This invention relates to control of the flow of feedwater to the steam generators in a pressurized water reactor (PWR) and in particular to a method and apparatus which avoids unnecessary actuation of the auxiliary feedwater system as a result of transients encountered following a reactor trip.
2. Prior Art
In a PWR nuclear power plant, reactor coolant, which absorbs the heat generated by fission reactions in the reactor core, is circulated through a steam generator to generate steam which drives a turbine-generator to produce electricity. Water for generating steam is provided by a main feedwater system which automatically maintains a programmed water level in the steam generator. Since it is essential that the heat generated by the reactor be removed, the main feedwater system is backed up by an auxiliary feedwater system which automatically takes over should the main system fail to maintain at least a minimum level. As opposed to the main feedwater system, the auxiliary system does not automatically regulate the flow of water to the steam generator to maintain the water level in accordance with a programmed schedule, since this system is a protection system with a minimum of control devices.
Under certain circumstances, a reactor trip can initiate a chain of events which leads to activation of the auxiliary feedwater system when there is nothing wrong with the main feedwater system. This is undesirable for four principal reasons. First, the water supplied by the auxiliary system is colder than that supplied by the main system and the PWR has a negative temperature moderator coefficient. This means that as the temperature of the reactor coolant is lowered and the coolant becomes more dense it slows down more neutrons to the thermal level required for fission, and hence the reactivity of the core increases. Thus, the cold feedwater in the steam generator can lower the reactor coolant temperature to the point where the shut down margin of the reactor becomes unacceptable. Secondly, the auxiliary feedwater system is an emergency system and should not be challenged unnecessarily. Thirdly, the excessive cooling imposes unnecessary thermal stresses on the equipment of the primary system. Fourthly, if the auxiliary feedwater system is badly controlled by the operator, the excessive cooling can result in the actuation of the safety injection system which is another emergency system which injects borated water into the primary circuit.
Accordingly, it is a primary object of the present invention to control the flow of main feedwater to the steam generators in a PWR following a reactor trip so as to avoid excessive primary cool down and unnecessary activation of the auxiliary feedwater system.